The present invention concerns a graphte side reflector in block construction for use in a gas-cooled high temperature reactor with a reactor core filled with spherical fuel elements with nose-like projections, the so-called nose stones, formed by graphite blocks, protruding radially into the reactor core, comprising regularly spaced slit-like recesses arranged in a grid pattern on their inside surface and a continuous gap on their frontal side, whereby the reactor core is connected with a cavity located vertically in the nose stone and serving to receive the absorber material.
The purpose of reflectors in nuclear reactors is to prevent losses due to the outward migration of neutrons. As indicated by the name, at least part of the neutrons moving to the outside are to be reflected back into the fission zone of the reactor, the so-called reactor core. The term "side reflector" designates the laterally placed reflector in contrast to the bottom and top reflector.
The material used for the reflectors in gas-cooled high temperature nuclear reactors comprises high purity graphite, which under neutron irradiation undergoes a change of its crystalline structure, manifested by volume and mechanical property variations.
Under the effect of temperature and high neutron fluxes, initially negative expansion appears in graphite, which, however, beginning at a point of reversal turn into positive expansion eventually exceeding the original dimensions. Differences in expansion within the part generate residual stresses as the result of differential flux and temperature distributions over its cross-section. In order to keep these residual stresses within allowable limits, free expansion must be made possible. This may be obtained advantageously by slit surface structures.
Recent developments in gas-cooled high temperature nuclear reactors, particularly those of low capacity (approximately 100 MWel) and corresponding small diameters, provide for the shutdown of the reactor in place of absorber rods inserted directly into the pile of spherical fuel elements, small absorber balls introduced into the corresponding cavities of the side reflectors. For this purpose, so-called nose stones are placed into the core. They include continuous vertical cavities to receiv the shutdown absorber balls. The nose stones are brick-shaped graphite blocks, physically bonded to the side reflector from which they are radially protruding into the reactor core while extending over the entire height of the latter.
Because of the afore-mentioned volume changes and the residual stress states created thereby in the irradiated graphite blocks, the surfaces on the core side of the latter are provided with vertical and horizontal surface slits, representing the resolution of the original large surface into small individual segments.
To control stresses in the nose stone, the cavities provided for the containment of the small absorber balls are connected by means of narrow, gap-like continuous openings with the core. Stresses in the nose stones are thereby reduced to a tolerable level. The afore-mentioned expansions (the so-called Wigner expansions) lead, however, in the course of the operation to variations of the gap width of a magnitude such that the separation of the absorber material and the fuel elements is no longer assured and small absorber balls may exit from the cavities, while fuel elements can enter them.
Based on this state of the art, it is the object of the invention to propose a design solution that may be manufactured simply and cost effectively, while avoiding the afore-mentioned difficulties.